The structure of the hull of a vessel is a complex array of plating and girders which must withstand a variety of external and internal forces while at the same time defining a commercially useful vessel form.
The two general categories of loading which the components of a ship must withstand are longitudinal bending, or the tendency of a hull to bend in a vertical plane due to differences between the longitudinal distributions of weight and buoyancy, and local bending caused by gravity, pressure of the sea and various liquids, fuels, equipment and cargo within the hull. Most structural parts of a ship or vessel are affected by either or both of these load categories.
The traditional element of structure used to construct the surfaces of a vessel is a steel plate stiffened by uniformly spaced parallel structural members. The spacing of these stiffeners is such to permit an acceptable local bending stress in the plate and is determined by the plate thickness and the pressure or load on the plate. The size of the stiffeners themselves depends on their spacing, plate loading and length, or span, between supporting members. These supporting members can be a perpendicular plane of plating or other structural members, larger than the stiffeners, which usually are perpendicular to the primary stiffeners. These deeper secondary members are generally referred to as webs. This type of traditional stiffened flat plate structure is complicated because careful hand construction is required at every intersection of primary and secondary members. Openings must be cut in the secondary webs to allow the primary stiffeners to pass through and clips are welded to both members to assure structural connection. The primary stiffeners are generally run fore and aft and are made continuous through the transverse webs and bulkheads. This is done so that in addition to stiffening the flat plate panels against local loading, they contribute their weight to the weight of the shell plating in providing resistance to the longitudinal bending described above. Generally, for larger vessels longitudinal bending is the predominant source of structural load while for the small vessels, local loading is more important.
In the early 1900's there was an attempt to modify the above referenced form of contruction, particularly in riveted forms of construction and examples of early patents are British Pat. No. 7792, 1913; British Pat. No. 20,479, 1912; British Pat. No. 26,036, 1910; and British Pat. No. 21,952 dated 1894.